The Moon

The Moon photographed by the crew of Apollo 17 in December 1972Image: NASA

The Moon is the closest astronomical object to the Earth. With the Earth it forms what is almost a double planet as no planet has a satellite which is as large in comparison to the size of the planet.

The Moon has a diameter of 3476 km and orbits the Earth at a mean distance of 384,000 km. It orbits the Earth in 27.322 days and always keeps the same face pointed towards the Earth.

The Moon shines by reflecting the light from the Sun and shows the characteristic phases during each orbit of the Earth. Near New Moon, when the sunlit portion of the Moon is small, the phenomenon of 'the old Moon in the young Moon's arms' is often seen. This is caused by sunlight being reflected towards the Moon by the Earth and being reflected back again to the Earth. We are seeing Earthshine, the equivalent of moonlight on the Earth.

The orbital plane of the Moon is inclined to that of the Earth about the Sun and so eclipses are only seen when New Moon or Full Moon occur when the Moon is near to the crossing points of these planes (see our fact file on eclipses).

The gravitational pull of the Moon and the Sun are responsible for the tides (see our fact file about the tides).

The Moon has no atmosphere. Any early atmosphere that the Moon might have had has escaped from the Moon's feeble gravitational pull. This is only one sixth that at the surface of the Earth. Because of the lack of any atmosphere the temperature of the Moon's surface varies between –180°C and +110°C. The Moon offers little protection from the solar wind, cosmic rays or micrometeorites and so it is not surprising that there is no form of life on the Moon.

Man in the MoonThe Moon's surface is characterised by light mountainous regions interspersed with dark maria. The 'Man in the Moon' is formed from patches of these two types of terrain. The maria are vast impact basins which have been filled with basaltic rocks some 3000 million years ago. Much of the Moon's surface is covered with craters. These are the result of impacts by meteors. The largest are about 200 km in diameter, the smallest are only about a metre across. Most of these craters were formed between 3000 and 4000 million years ago.

Much of our knowledge of the structure of the lunar surface and the geology of the Moon comes from the landings of the Apollo series and the samples of lunar material brought back to Earth. Despite this we are still not sure how the Moon was formed. In recent years, the most widely-accepted theory is that the Earth was struck by a Mars-sized body early in its history. Part of the resulting debris then coalesced into the Moon.

Viewing the MoonThe Moon is probably the most satisfying object to look at through any telescope. The craters and mountains can be seen with even a small telescope. The best place to look is near the terminator, where the Sun is either rising or setting. Here the shadows cast by mountains and crater walls are longest and can give very dramatic views. After as short a time as an hour changes in the shadows can be seen as the sunlight reaches or leaves peaks near the terminator.

A composite of the south pole of the Moon taken by Clementine.Image: Naval Research LaboratoryClick here to view a high-resolution PDF

Recently two probes completed successful missions to map and determine the composition of the Moon. The Clementine probe mapped the surface over two months in 1994 and hinted at the presence of water ice in some of the permanently shadowed craters at the poles.

Lunar Prospector found strong evidence for up to 300 million tons of water ice deposits at both poles as well as mapping lunar magnetic fields and determining the abundances of many elements.

Transient lunar phenomenaMany amateur astronomers look for 'transient lunar phenomena' (TLPs). These are outbursts of some kind which give rise to short-lived colour or brightness changes in small areas. It is not clear how many of these are real or what causes them.

During the Clementine mission, observations of TLPs by amateur astronomers were followed up by direct imaging by the orbiting spaceprobe. On 23 April 1994, there were reports of an obscuration over the so-called Cobrahead feature on a region called the Aristarchus plateau. Clementine images taken on 3 March and again on 27 April do indeed show a change as part of the region is a slightly different colour.

Confirmation of this tentative discovery would be very significant as the Aristarchus region is one of the youngest regions of the Moon. The Cobrahead feature is a collapsed lava tube that came from a volcano that had its heyday billions of years ago. It is also a region where TLPs have been seen in the past. Perhaps pockets of gas seep up through the ground and, when caught by the bright rays of the lunar dawn, glow in reds and blues. Or perhaps heating effects cause sub-surface explosions.

More facts...

Four times smaller in diameter than the Earth, our Moon was probably formed shortly after the rest of our solar system, about four and a half thousand million years ago.

How big is the Moon?The Moon has a diameter of about 3467 kilometres (km).

How far away is the Moon from Earth?Because the Moon orbits the Earth in an elliptical (oval) shape, it is sometimes closer than at other times. The distance between the centre of the Earth and the centre of the Moon varies between 356,400 km and 384,400 km.

How long does it take the Moon to orbit the Earth?The Moon takes 27.3 days to go around the Earth once. This is exactly the same time that it takes for the Moon to rotate once on its axis. Because these rates are equal, the same face of the Moon always ends up pointing towards the Earth.

Does this mean that we never see the back of the Moon?From the Earth we always see the same face of the Moon. The back of the Moon can only be observed from space. The first photos of the back of the Moon were taken in 1959.

Why do things weigh less on the Moon?Things have weight because of the pull of gravity. Gravity is the force of attraction that exists between any two objects. The more massive the objects, or the closer together they are, the greater the force of gravity. Normally we only notice the force if one of the objects is very massive. Although the Earth and Moon are both very massive, the Moon is smaller, so its force of gravity is less. Any object on the Moon will weigh about six times less than it does on Earth.

Has anyone ever been to the Moon?Yes. Americans have landed on the Moon six times. The first time was in July 1969 and the last time was in December 1972.

Why does the Moon appear to change shape?The appearance of the Moon changes over a 29.5 day cycle. We can only see the Moon because it is reflecting light from the Sun. The side of the Moon which is facing the Sun is always lit up. From the Earth, we can see all the lit face when there is a full Moon, but only half of it a week later. The amount of the lit face that we can see depends on the position of the Moon in its orbit.

Why is there a new Moon every 29 days and not every 27.3 days?At the same time as the Moon is orbiting the Earth, the Earth is also orbiting the Sun. This means that the Moon has to move through an angle which is slightly greater than °360 to bring it back between the Earth and the Sun.

What is an eclipse?If the new Moon passes exactly between the Earth and the Sun it prevents the Sun's light from reaching a small part of the Earth's surface for a few minutes and an eclipse of the Sun occurs. Outside this area, a partial eclipse rather than a total eclipse is seen – only part of the Sun being blotted out.

In the same way, the full Moon sometimes passes exactly behind the Earth, so that the Earth prevents the Sun's rays from reaching the Moon directly. When this happens, the Moon goes very dim and an eclipse of the Moon has occurred.

When can I see an eclipse?The next total solar eclipse of the Sun will not be visible from the UK mainland until 23 September 2090, although one will be seen from the Channel Islands on 3 September 2081. The last total eclipse of the Sun visible from this country was at about 11.00 on 11 August 1999. The table below lists all the eclipses that occured between the middle of 1997 and the year 2000.

Eclipses of the Sun and the Moon

Date

Type

Visible from the UK?

1-2 Sept 1997

Sun (partial)

No

16 Sept 1997

Moon

Yes.Begun at dusk and finished before 21.30

26 Feb 1998

Sun

No

21–22 Aug 1998

Sun (annular)

No

16 Feb 1999

Sun (annular)

No

28 July 1999

Moon

No

11 Aug 1999

Sun

Yes.Total from parts of Devon, Cornwall &Alderney. Partial from elsewhere.

21 Jan 2000

Moon

Yes.Occured in the early hours, after 03.00

5 Feb 2000

Sun (partial)

No

1 July 2000

Sun

No

16 July 2000

Moon

No

31 July 2000

Sun (partial)

No

25 Dec 2000

Sun (partial)

No

What is an annular eclipse?Neither the Moon nor the Earth have perfectly circular orbits. The distance from the Earth to the Sun and from the Earth to the Moon are therefore continuously changing. The further away from the Earth the Sun or the Moon are, the smaller they appear. Although the moon is much much smaller than the Sun, it is also much closer.

When seen from the Earth, both the Sun and the Moon appear roughly the same size. When the Sun is closer to the Earth, it will appear very slightly bigger. When the Moon is further from the Earth it will appear very slightly smaller. If the apparent size of the moon is less than the apparent size of the Sun when it passes in front of it, the moon is unable to block the light from the Sun completely, and an annular eclipse occurs. What is seen is a bright ring with a dark centre.

How long do eclipses last?The whole process usually takes several hours. When a total eclipse of the Sun occurs, the whole of the Sun will be blocked for no more than a few minutes. On the other hand, when an eclipse of the Moon occurs, the Earth can be completely in the way for over an hour. This is because if viewed from the Sun, the Earth would appear about four times bigger than the moon.

Why isn't there an eclipse of the Sun or the Moon every month?You might think that there would be an eclipse every time there was a full or a new Moon. However, the Moon's orbit is tilted at a slight angle. On most occasions when the new Moon is between the Sun and the Earth, it passes unseen above or below the Sun without blocking the Sun's rays to the Earth. Similarly, when there is a full Moon and the Earth is between the Moon and the Sun, the Moon is too high or too low for the Earth to block the light from the Sun.

What has the Moon got to do with the tides?On the Earth, the level of the oceans changes in a regular pattern throughout the day. These changes in level are called the tides. The gravitational pull of the Earth, and to a lesser extent the Sun, causes the tides to occur. As the Earth spins on its axis, a bulge of water, or high tide, appears on the side facing the Moon. At the same time another bulge appears on the opposite side of the Earth. The Earth takes 24 hours to spin round once on its axis. If the Moon was not going round the Earth, high tides would occur every 12 hours. The movement of the Moon around the Earth means that the average time between one high tide and the next is 12 hours and 25 minutes.

Does the Moon have an atmosphere?No, there isn't enough gravity to keep one. As there is no atmosphere, there is no wind or rain to alter the surface of the Moon either.

What does the surface of the Moon look like?The surface of the Moon is covered in craters. These were formed by meteorites striking the surface of the Moon. Most of them were formed very early on in the Moon's life. They can be seen by observing the Moon with binoculars. The time when they are most visible is at first and last quarter when the shadows caused by the craters are fairly long, and therefore more visible.